Currently, in an outgoing cellular deployment, suitable powered macro cells have been deployed to cover larger areas. However, the deployment of such macro cells does not manage to abate quick capacity degradation as the number of user equipment (UE) operating in the macro cells coverage areas increases. Therefore, cellular operators are now reinforcing the macro cells deployment along with one or multiple low powered small cellular cells (termed as Femto/Pico/Micro cells) placed at multiple strategic locations within one or more macro coverage areas and such arrangement of the macro cells along with the small cellular cells is termed as a Heterogeneous Networks or HetNets.
In a typical HetNet, the strategic locations for small cells generally include areas with high density of users, such as shopping malls, airports, railway/bus stations, colleges, etc. Also, these locations might include area with dead-spots, or areas with low macro cell signal strengths, such as indoor establishments or peripheral locations of the macro cell coverage area. The placement of the small cellular cells along with macro cells at multiple strategic locations as described above, results in HetNets that not only provides an increased mobile data capacity but also provide better mobile coverage and thereby enhancing the overall mobile broadband experience of the user.
In recent years, Wi-Fi technology based on IEEE 802.11 standards has also undergone tremendous growth and commercialization. Almost all available UE with cellular capability support have now Wi-Fi capability by default in order to connect to available Wi-Fi networks operating in the unlicensed frequency bands such as 2.4 GHz, or 5 GHz. Therefore, with technological augmentation, the cellular operators use universal and cost effective Wi-Fi technology in pursuing the HetNet deployment and implementation strategy. Many operators are now deploying low powered Wi-Fi cells along with cellular small cells at multiple strategic locations identified for the HetNet. Further, for ease of maintenance and provision, a few operators are also using Wi-Fi integrated versions of small cellular cells, wherein the Wi-Fi and the cellular small cell technology are made available on common equipment.
An exemplary case of a typical heterogeneous network comprises a macro base station for providing wide area coverage to service users. Within the macro cell coverage, several low power nodes are employed in service areas having a higher density of users requiring high data rates. Examples of such low power nodes comprise a first microcell and a second microcell in which the first microcell and the second microcell integrated with Wi-Fi radio are also used widely to provide multi technology hotspot capacity/coverage goals. The operators could also deploy sovereign & cost effective Wi-Fi Access points in hotspot areas to offload cellular load, and to meet capacity/coverage requirements of the users.
In the HetNet, the macro base station coverage could be used for wide area overlay mobility coverage, while micro base stations along with Wi-Fi Access points coverage could be used for mobile capacity upgrade. In a typical cellular deployment, where cellular cells are based on long-term evolution (LTE) specifications, laid out by the 3rd generation partnership project (3GPP), each cellular cell (Macro/Micro/Pico/Femto) is identified by a physical cell identity (PCI) number at a radio layer. The PCI is used exclusively at radio layer procedures including neighboring cells measurements and reporting functions. At higher levels, a global cell identity (ECGI) is generally used to identify a cellular cell for scenarios, such as handover routing.
However, the total numbers of PCIs are limited in the LTE specification. For the LTE, primary and secondary synchronization signals together encode the different PCI of a LTE cell. The primary synchronization signal provides physical layer identity (0, 1, and 2) while the secondary synchronization signal provides the physical layer cell identity group (0, 1, 2 . . . 167). The total number of PCIs, in the LTE, is therefore limited to 504.
As there are a limited total number of PCIs in the LTE, the PCI allocation becomes a challenging task in the cellular deployment. Further, each cellular cell in the LTE system is required to be assigned a PCI that is different from the PCI of the neighbouring cells, so as to avoid PCI collision and PCI confusion scenarios in such cellular deployment.
The PCI collision occurs when two neighboring cells with overlapping coverage area share the same PCI. This is a serious problem since the user equipment in that overlapping area cannot distinguish between signals coming from the two neighbouring cells and thereby causing a loss of processing gain, synchronization issues, and high decoding errors.
The PCI confusion occurs when a PCI reuse happens among the neighbouring cells of the same cell. This leads to cell identification problem, where a serving cell is unable to uniquely identify the neighbour base stations by identifying the corresponding PCI associated with each neighbour base station. Thus, in an event a UE moves towards one of these neighbour base stations, the macro cell is unable to initiate a handover to the correct neighbour base station/cell.
Further, since there exists a limitation in terms of quantum of PCI space, the PCI allocation of every cellular cell whether macro or small cell becomes even more challenging in the HetNet deployment. Further, it is evident from the HetNet deployment that the PCI space could be reused among the small cell clusters within the macro coverage area since these are separate from each other; however, such a scheme of reuse could result in the PCI confusion state when the UE being served on the macro cell initially is being handed over to a small cell which is a part of small cell(s) cluster deployment under the macro cell coverage area.
Furthermore, there exists several known solutions to resolve the PCI confusion situations. A solution for resolving the PCI confusion is to let the UE provide more information to the serving base station enabling it to uniquely identify the target cell, for example by applying principles similar to automatic neighbour relation (ANR). One such procedure is mentioned in 3GPP Technical Specification 36.300, and is called “inbound mobility to E-UTRAN CSG cells”, where the UE is asked to read the E-CGI from the target neighbor cell to resolve a possible PCI confusion situation that arises while performing the handover to the target neighbor cell.
However, the frequent use of such ANR based procedure(s) may create interruption in the ongoing transmissions in the serving base station as UE requires long gaps to read E-CGI from the target cell. Also, it could possibly delay the handover process as the UE needs to fetch additional information from the target cell.
Moreover, with reference to related art, U.S. Ser. No. 14/115,810, such PCI confusion situation(s) is dealt by conveying extra information element (containing identifiers related to handover, such as cell radio network temporary identifier (C-RNTI), security settings, etc.) in the handover request message(s) to all the target base stations involved in the PCI confusion situation so that those target base stations send response message(s) with identical handover identifiers (as mentioned in the request message) to the serving base station. The serving base station thereafter would send one unique handover command (with commonly allocated handover identifiers known to all the target base stations (‘Cell2’ and ‘Cell3’) involved in PCI confusion situation) to the UE enabling it to perform handover to appropriate target base station. However, the proposed method requires overhead of maintaining a reserve space of handover identifiers (such as cell radio network temporary identifier (C-RNTI)) which are to be allocated by the serving base station to deal with PCI confusion situations. Further, since all the target base stations (i.e. the first micro cell and the second micro cell) are prepared for the handover in an event the PCI confusion arises, there is overhead of signalling towards unrequired target base station(s).
Accordingly, existing technologies do not solve the problems related to resolution of the PCI confusion arising in the HetNet during handover procedures so as to reuse the PCI space among various clusters of small cells operating under a designated macro cell coverage area.